<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;padding: 0;
            box-sizing: border-box;
        }
        .box{
            height: 2.5rem;
            position: relative;
            /* box-sizing: border-box; */
            /* background: green; */
            /* border-bottom: 1px solid #000; */
            /* margin-bottom: 10px; */
        }
        .box::after{
            content: '';
            height: 1px;
            /* css像素 */
            /* 物理像素 */
            position: absolute;
            bottom: 0;
            left: 0;
            right: 0;
            background: #000;
            transform-origin: 0 100%;
            transform: scaleY(50%);
        }
    </style>
</head>
<body>
    <div class="box"></div>
    <div class="box"></div>
</body>
</html>